In recent years, there has been much interest in light-emitting diodes (hereinafter referred to as LEDs) as illumination and light generating means capable of being made lightweight, thin and with low power consumption. LED devices are devices that emit light when a forward current is passed through the pn junction of a semiconductor, produced using III-V semiconductor crystals such as GaAs and GaN. Progress in techniques of semiconductor epitaxial growth and light-emitting device processing has resulted in development of LEDs of exceptional conversion efficiency, that are widely used in various fields.
LED devices are composed of a p-type layer and an n-type layer formed by epitaxial growth of III-V semiconductor crystals on a monocrystalline growth substrate, and an optically active layer sandwiched in between. Generally, LED light-emitting devices are formed by epitaxially growing a III-V semiconductor crystal such as GaN on a growth substrate such as monocrystalline sapphire, then forming electrodes (Patent Document 1).
Recent years have seen rapid advances in the emission efficiency of LED devices, and the higher luminance of LEDs has been accompanied by increased heat generation. For this reason, the reliability of LEDs can be reduced in the absence of adequate heat dissipation measures. Specifically, increases in LED device temperature can result in problems such as reduced luminance and shorter device lifespan. Therefore, metal materials of high thermal conductivity such as copper and aluminum are used for the substrate portions where LEDs are mounted, in order to increase the heat dissipating ability of LED packages. If the substrate alone provides inadequate heat dissipation, metallic heat sinks are sometimes further used as heat dissipation measures.
In accordance with the trend toward application of LED devices to illumination applications, LEDs are becoming increasingly larger and more powerful. While LED devices are generally used by bonding them to substrates by soldering or the like, differences in thermal expansion between the LED devices and substrate materials can lead to stresses generated at the junction layer, which in the worst case can result in destruction of the LED devices or extremely reduced reliability.
In order to cope with the increased heat generation due to the increased power and size of LED chips, metal matrix composites formed as a composite of ceramic particles and metallic aluminum are known as materials having high thermal conductivity and low thermal expansion coefficient (Patent Document 2). For example, a metal matrix composite formed as a composite of aluminum and silicon carbide satisfies the above-described properties, but the material is difficult to work with and is rather expensive for use as a substrate for LEDs. For this reason, metal matrix composites formed as a composite of aluminum with graphite have been considered as metal matrix composites that are relatively easily worked (Patent Document 3).
Metal matrix composites of aluminum and graphite were initially developed for the purpose of forming sliding elements. They are being studied for the possibility of improving their properties by infiltrating the aluminum alloy with graphite materials at high temperatures and high pressures (Patent Document 4).    Patent Document 1: JP 2005-117006 A    Patent Document 2: JP 3468358 B    Patent Document 3: JP 3673436 B    Patent Document 4: JP H5-337630 A